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The properties of this mineral, as they appear under the ore microscope, have not been described previously. There is little question that the mineral is identified with greater ease by thin section, but as it is so commonly found in these wolfram ores, a knowledge of its properties under reflected light assists in the study of its relations with the other ore minerals. Its relation to wolfram is much more clearly seen in polished than in thin sections.
Scheelite takes an excellent smooth polish with no pits remaining. Hardness: D-, less than wolfram, the latter standing up slightly in relief from it. Reflectivity: Reflectivity about 10, just less than cassiterite, and difficult to compare owing to the latter's uneven surface, but best judged by strong screening. Its reflectivity is greater than that of any gangue mineral present. Anisotropism: completely obscured by the pale yellow and white inner reflection.
Scheelite shows two modes of occurrence, in one (much the more abundant) it replaces wolfram, and in the other it is interstitial to the latter. Sometimes the one is optically continuous with the other mode of occurrence, their reflectivity being identical, but the replacing scheelite may show a pale yellow inner reflection in contrast with the white of the interstitial scheelite.
Negative.-HCl, HNO3, aqua regia, H2SO4, H2O,
Positive.-No reagent yet tried. Microchemical tests:-The mineral is fused as a minute bead with sodium carbonate and sodium peroxide, crushed on a glass slide and dissolved in a drop of distilled water. The solution is removed by a capillary tube and dropped on to a filter paper moistened with a drop of conc. HCl, then a drop of KCNS and SnCl2 solution added, the typical blue colour of W developing. The test for Ca is scarcely necessary but can be done on the residue from the water solution. This residue is taken up in HCl, a drop of ammonium sulphide added, warmed, a drop removed by capillary tube to a fresh place on the slide, and a fragment of dihydroxy tartaric oxazone added, a white precipitate forming, but sometimes only after a lapse of several minutes.
Pyrite. This mineral is fairly widely scattered throughout these
Usually in small grains, it frequently has quite well defined
crystal boundaries. In one specimen from near the junction of the Paunglaung Chaung and the Angyi Chaung, on the high ground north-east of Sheet No. 94 A/5, the pyrite has zonal layers which possess a slightly different colour, more creamy brown than the pale yellow of the pyrite. These layers are of just slightly lower hardness than pyrite, and have the same reflectivity and are isotropic. Etch tests are as for pyrite and microchemical tests were negative except for Fe and S.
Arsenopyrite.--Almost equalling pyrite in abundance, arsenopyrite is found in these ores to assume two forms. For the most part it occurs as rather coarse irregular grains, but in rare instances small needles of the mineral have been observed in chlorite (Pl. 8, fig. 3). The coarse grains commonly show twinning. A consistent excellent sulphur reaction indicates that löllingite is not present.
Molybdenite.-Flakes of molybdenite are commonly scattered throughout these ores. No example was observed of a molybdenite flake being enclosed in either a cassiterite or a wolfram crystal, but it is commonly interstitially arranged with respect to these minerals. It is often enclosed in quartz and in such sulphides as bismuthinite and galena (Pl. 9, fig. 3). Veinlike lines of molybdenite are invariably found to consist of a single contorted flake. Very commonly it occurs at the boundary between two minerals, but in such cases it appears that the original host mineral of the molybdenite has been replaced by later sulphides on one side of the flake.
Molybdenite polishes rather well notwithstanding its low hardHardness: B, the softest of all the ore minerals. Reflectivity varies widely, 30 along the basal direction (O), 15 in the direction of ć (E). This high reflection pleochroism is one of the diagnostic features of molybdenite. Colour: white and dark grey, according to direction. Anisotropism: very strong, white with pinkish tint. No internal reflection.
Etch tests-Negative to all reagents. HNO3 doubtfully positive.
Microchemical tests:-The test for Mo may be done exactly as for W, but the edge of the drop is red instead of the blue centre of W. Owing to the relative solubility of MOS, the mineral may be taken direct into solution with HCl instead of fusing first with a flux.
Galena. Although scattered throughout these ores, galena is not abundant. In specimens polished on cloth, minute veinlets of galena in cassiterite are liable to be overlooked owing to the extreme difference in hardness of the two minerals, particularly if the cassiterite surface is deeply pitted and grooved.
Sphalerite. Occasional coarse grains of sphalerite are seen, usually containing minute ex-solution droplets and veinlets of chalcopyrite and rarely of stannite. A very slight anisotropism, seen in a few grains, suggests that wurtzite is also present.
Sphalerite is very similar indeed to wolfram at first place under reflected light, as their colour and reflectivity are almost identical. They are, of course, readily identified under crossed nicols, and by the common association of chalcopyrite droplets in sphalerite.
Chalcopyrite.-Coarse grains of this mineral are relatively rare, but ex-solution droplets and veinlets in sphalerite are very abundant. Droplets of the mineral also occur in stannite.
Bismuthinite.-Most of the bismuthinite occurs as a fine intergrowth in galena, but occasional coarse grains have been determined. The optical and etch properties of this mineral were confirmed by micro-chemical reactions for Pb, Bi and S. No native bismuth was detected in these ores.
Covellite.--In addition to the covellite which had clearly replaced stannite and chalcopyrite, there are a few minute specks of covellite which cannot definitely be regarded as supergene, and may be hypogene.
Chalcocite. This mineral, like covellite, replaces chalcopyrite, but is not common. All the chalcocite noted in these ores is definitely supergene.
Cerussite. The galena has only very rarely been altered to cerussite. These three last supergene minerals usually occur together.
Tungstite. The wolfram has occasionally been altered to a WO, ochre, a brown powder which sometimes has a structure reminiscent of wood-tin.
Quartz. Easily the greater part of these veins consists of coarse white quartz. There does not appear to have been any considerable silicification of the country rock. Like wolfram, the quartz often
grew inwards from thin selvages along the vein walls. A few small well developed crystals of quartz occur, with interstitial calcite, in the centre of the lodes.
Calcite. In certain parts of the lodes calcite is the principal gangue mineral. It is white in colour with a remarkable silky sheen on the cleavage faces, and has a peculiar platy or almost acicular habit (Pl. 12, fig. 4); it readily disintegrates to a fine powder on rubbing. Under the microscope it is seen to be considerably crushed. Cassiterite occurs in this material as irregular crystals which fall away from their matrix on being touched. To obtain a polished section of such material thorough impregnation with bakelite resinoid was essential. The habit of this calcite is so unusual that its diagnosis was confirmed by analysis. A certain amount of fine quartz is present in each specimen, as well as a little sulphide; such quartz is irregularly replaced by the calcite (Pl. 12, fig. 3). The analysis was done on sulphide-free material :—
Analyst-Babu Mahadeo Ram.
Tourmaline. In the veins, both blue and brown tourmaline occur, but the blue variety is easily the more abundant. both as fine and coarse grains, and frequently shows quite well developed crystal faces. It is commonly associated with chlorite and muscovite, which may replace it (Pl. 10, fig. 4). A selvage which is sometimes found along the vein walls consists entirely of tourmaline.
Muscovite. Including lepidolite and gilbertite. These These are not particularly abundant in the vein material, and occur more particularly on the vein wall. The lepidolite shows the typical rose pink colour, whilst the fine-grained gilbertite is of a silvery white to silky green colour; both sometimes occur as radiating rosettes.
Coarse muscovite is very rare in the specimens examined; a curious pale greyish brown variety has been observed.
Chlorite. A little chlorite occurs in the gangue, usually replacing and veining other minerals, but is more abundant in the vein walls.
Fluorite. Although not detected in the more simple quartz veins, fluorite appears to be quite well distributed where the vein constituents are more complex, particularly where calcite, zoisite, garnet and beryl occur. It can be determined in the hand specimen. by its usually distinctive purplish colour, although a green variety is sometimes seen, and its refractive index provides a rapid confirmatory test.
Beryl. In certain places beryl is particularly abundant, and locally constitutes the greater part of the lode material. Once determined, it is readily distinguished in the hand-specimen. Usually it forms quite coarse crystalline aggregates with a typical pale blue colour. It is a variety low in alkalies, as suggested by its low refractive index, 1.576002 and w=1.583002.
Phenacite. This rather rare beryllium silicate was detected in these ores under the microscope. In thin sections it is colourless and shows an excellent prismatic cleavage, the mineral being uniaxial, positive, -1.650, -1.665. Some of the isolated mineral gave a good Be test.
Garnet.-A pale yellow garnet, apparently grossularite, was detected in two specimens from these veins. It occurs in a dense, banded vein material, one of the bands consisting almost entirely of grossularite and replacing calcite. The refractive index of the garnet is 1-745, and its specific gravity is 3.4.
Zoisite. Quite abundant zoisite was detected in one specimen. The country rock of the vein at this point is marble, and it is probable that much of the vein here has replaced the country rock. The specimen consists of coarse orthoclase-cassiterite on one side, and of orthoclase, zoisite, fluorite and calcite on the other.
Orthoclase. Occasionally orthoclase occurs as a principal constituent of the vein material, both in granite and marble country rock. It differs from the orthoclase of the granite in that it is coarse-grained and usually clear, although sometimes kaolinised. It is, however, replaced in turn by coarse zoisite, fluorite and calcite the presence of these necessitated a refractive index determination to distinguish it from calcic plagioclase.